1. Field of the Invention
The present invention relates to ink jet head, drive method thereof, and ink jet recording apparatus for causing ink to fly across a space with the help of deformation of an oscillating plate by means of electrostatic force.
2. Description of the Related Art
An ink jet type recording apparatus causes an ink drop to be ejected from a nozzle and thrown across a space to land on a recording medium to form an image. A number of ink jet heads have been invented to be used on such a recording apparatus. As an example, Japanese laid-open Patent Publication, JP-A-05-50601, discloses an ink jet head that uses an electrostatic actuator that ejects ink with the help of deformation of an oscillating plate by electrostatic force.
This electrostatic ink jet head has a laminar structure composed of three members joined together, i.e., a channel plate provided with a plurality of recesses, a glass substrate positioned opposite to the bottom surface of the recesses, and a cover plate. The recesses form a nozzle that ejects an ink drop, an ink channel that communicates with the nozzle, an oscillating plate that changes the pressure in the ink channel, a common ink chamber where ink is stored, and an inlet that serves as an ink supply port from the common ink chamber to the ink channel. The bottom wall of the ink channel forms an oscillating plate to generate the pressure for ejecting ink. A plurality of ink channels and nozzles are provided in accordance with the number of dots to be printed at a time. Moreover, a first electrode is provided on the surface of the side which is not facing the ink channel, or on the backside of the oscillating plate. A second electrode is provided on the surface of the glass substrate opposing the first electrode separated by a small gap from the first electrode. The recesses provided on the backside of the oscillating plate and the top surface of the substrate also serves as the members to form these electrodes.
The electrostatic type ink jet ejects ink based on the following operating principle to form an image on the recording medium.
First of all, when a voltage is applied between the first electrode and the second electrode by means of a drive circuit, an electrostatic force is generated between the electrodes. Accordingly, the oscillating plate deforms by being drawn in the direction toward the second electrode, or in the direction of moving itself away from the ink channel, which is communicating with the nozzle. At this time, the volume of the ink channel increases. Therefore, ink is drawn through the inlet into the ink channel to fill it up. Next, the application of the voltage between the first and second electrodes as opposing electrodes is terminated and the charges are discharged. The oscillating plate returns to its original position by the restoring force due to its own rigidity. In the mean time, the oscillating plate sharply compresses the volume of the ink channel to generate a pressure. Consequently, the ink stored in the ink channel is ejected, flies across a space, and lands on the recording medium to form an image.
Such an electrostatic jet head has an advantage of allowing us to realize a higher density constitution and to print using a relatively low voltage in comparison with the method of ejecting ink using the deformation of a piezoelectric device.
However, it is necessary to make the gap between the first and second electrodes extremely small in order to reduce the applied voltage on an electrostatic ink jet head. For example, the electrode gap is set to about 0.3 .mu.m. Consequently, it is necessary to produce and assemble individual components such as the channel plate with extreme accuracy in manufacturing the head.
Moreover, since the electrode gap is only about 0.3 .mu.m, there is a danger of causing a short circuit between the first electrode as an individual electrode and the second electrode as a common electrode when the oscillating plate is oscillated. In order to prevent the short circuit, a protective layer can be provided on top of the electrode. However, a problem with the protective layer is that it is susceptible to chronological changes.
The pressure P generated by the electrostatic ink jet head can be expressed by the following formula: EQU P=1/2.multidot.{.di-elect cons..sub.r.multidot..di-elect cons..sub.o.multidot.(V/G).sup.2 }
wherein the symbol .di-elect cons..sub.r denotes the relative dielectric constant between the opposing electrodes, the symbol .di-elect cons..sub.o denotes the dielectric constant in vacuum, 8.8.times.10.sup.-12 [F/m], the symbol V denotes the applied voltage [V], and the symbol G denotes the distance between the electrodes [m].
In case of the conventional electrostatic type ink jet head, air is inserted, whose relative dielectric constant is 1, in the gap between the opposing electrodes. Therefore, it is necessary to set the gap G between the electrodes as small as 0.3 .mu.m as mentioned above in order to generate a sufficient pressure to cause ink to fly using, for example, a drive voltage of 40V. The manufacture of such a head has a problem that it requires high precision machining and assembly practices.
Incidentally, the electrostatic type ink jet head uses a constitution of causing the ink in the ink channel to be ejected and fly by means of deforming the oscillating plate as mentioned above. Therefore, a situation can occur, in which the mechanical constitution can no longer follow the demand when the drive frequency of the ink head, or the frequency of the voltage applied between a pair of electrodes is increased in case of continuous printing. Thus, the ink jet head drive frequency is limited by the natural frequency of the ink in the ink channel and the natural frequency of the oscillating plate.
Therefore, a problem has been noted that there is a limit to the improvement of the head response by means of increasing the drive frequency of the ink jet head.